A superabsorbent polymer (SAP) is a type of synthetic polymeric material capable of absorbing moisture from 500 to 1000 times its own weight. Various manufacturers have denominated it as different names, such as SAM (Super Absorbency Material), AGM (Absorbent Gel Material), etc. Since such superabsorbent polymers started to be practically applied in sanitary products, now they have been widely used not only for hygiene products such as disposable diapers for children, etc., but also for water retaining soil products for gardening, water stop materials for civil engineering and construction, sheets for raising seedling, fresh-keeping agents for food distribution fields, materials for poultices, or the like.
Such superabsorbent polymers may be prepared by polymerization, drying, pulverization, sorting, and surface-crosslinking processes. Upon surface-crosslinking, a solution including a surface-crosslinking agent is generally sprayed onto the pulverized polymer. In this regard, particle size distribution of the superabsorbent polymer is important.
Due to a surface area difference depending on particle size, non-uniformity may be caused by the surface treatment solution. Therefore, to minimize distribution of the surface treatment solution, a powder is controlled to have a particle size of 150 to 850 μm and then surface treatment is performed. Further, a powder having a particle size of smaller than 150 μm is not subjected to surface treatment, but is recycled to a polymerization reactor or a monomer solution, or another process.
However, a product of 150 μm or smaller formed during pulverization, in particular, a fine powder of 150 μm undergoes an additional crosslinking reaction at a primary drying temperature due to the surface-crosslinking agent remaining during recycling, and thus undergoes a secondary surface-crosslinking reaction during the surface-crosslinking process, resulting in deterioration of physical properties.